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Author: Hon A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand, 1997. Updated September 2015.
Scabies is a very itchy rash caused by a parasitic mite that burrows in the skin surface. The human scabies mite's scientific name is Sarcoptes scabiei var. hominis.
Scabies affects families and communities worldwide. It is most common in children, young adults and the elderly. Factors leading to the spread of scabies include:
Scabies is nearly always acquired by skin-to-skin contact with someone else with scabies.
Typically, several scabies mites infest an affected host. After mating, the male mite dies. The female scabies mite burrows into the outside layers of the skin, where she lays up to 3 eggs each day for her lifetime of one to two months. The development from egg to adult scabies mite takes 10–14 days.
Scabies causes a very itchy rash. It’s essential to search for burrows carefully in a patient with a severe itch, especially if the rash is mild. Contacts should be examined for burrows, whether or not they are itchy.
Scabies burrows appear as 0.5–1.5 cm grey irregular tracks in the web spaces between the fingers, on the palms and wrists. They may also be found on or in elbows, nipples, armpits, buttocks, penis, insteps and heels. Dermatoscopic or microscopic examination of the contents of a burrow may reveal mites, eggs or mite faeces (scybala).
Scabies rash is a hypersensitivity reaction that arises several weeks after initial infestation. It has a varied appearance.
Secondary infection is due to scratching and the effect of the mite on the skin's ability to fight bacteria.
Crusted scabies (previously called Norwegian scabies ) is a very contagious variant of scabies in which an individual is infested by thousands or millions of mites living in the surface of the skin.
Risk factors for crusted scabies include:
A case of crusted scabies is the usual reason for an outbreak of scabies in an institution. Patients and staff in the institution may present with:
The clinical suspicion of scabies in a patient with an itchy rash, especially when reporting itchy household members, can be confirmed by:
Crusted scabies reveals numerous mites on dermatoscopy or microscopy, raised immunoglobulin E (IgE) and eosinophilia.
Consensus criteria for the diagnosis of scabies were published by the International Alliance for the Control of Scabies (IACS) in 2018 .
A: Confirmed scabies is diagnosed if there is at least one of:
A1: Mites, eggs or faeces on light microscopy of skin samples
A2: Mites, eggs or faeces visualized on an individual using a high-powered imaging device
A3: Mite visualised on an individual using dermoscopy.
B: Clinical scabies is diagnosed if there is at least one of:
B1: Scabies burrows
B2: Typical lesions affecting male genitalia
B3: Typical lesions in a typical distribution and two history features.
C: Suspected scabies is diagnosed if there is one of:
C1: Typical lesions in a typical distribution and one history feature
C2: Atypical lesions or atypical distribution and two history features.
History features are:
H2: Close contact with an individual who has an itch or typical lesions in a typical distribution.
Careful attention to instructions is essential if scabies is to be cured.
The chemical insecticides used to treat scabies are called scabicides. The scabicide is applied to the whole body from the scalp to soles. The usual topical treatment is 5% permethrin cream, left on the entire skin for 8–10 hours. It should be applied under fingernails using a soft brush.
Oral ivermectin 200 mcg/kg is convenient, but more expensive than topical permethrin. It may be slightly less effective. It is mainly used for mass treatments in institutions, or in patients unable to use topical therapy.
Gamma benzene hexachloride cream is no longer recommended or available due to neurotoxicity; it is also a suspected carcinogen and is no longer marketed in New Zealand. Other proven treatments include:
Treatment should be repeated after 8–10 days after the first application to catch mites that have newly hatched. Crotamiton cream can be used to reduce itch; it is a weak scabicide.
Patients with crusted scabies may need repeated oral and topical treatments over several weeks or longer.
Contacts must be identified and treated. In addition:
Scabies itch and rash are expected to improve within a few days of successful treatment and to completely clear within a month.
A rash may persist after scabies treatment. Reasons for this include:
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